The application of the red, green and blue components of the luminescent materials comprising rare earth metals developed for achieving a higher lumen/watt (lumerg) efficiency in the envelope is not difficult by the commonly used wet methods. The components may be applied in the desired ratio to the inner wall of the discharge vessel by suitably adjusting the viscosity and density of the coating suspension. Although carbon or carbonized organic substance contaminations having a disadvantageous effect on the efficiency of the luminescent material and the gas content of the discharge tube can remain even after the usual heat treatments when the luminescent material is applied as an organic solution by using nitrocellulose as binding agent, the disadvantageous effects occure more frequently when the luminescent material is applied in the form of an aqueous solution. The removal of the binding agent is also more difficult compared to the removal of nitrocellulose, when the luminescent material is applied in the form of an aqueous solution.
The discharge tubes of 26 mm diameter of the energy-saving fluorescent lamps are generally coated by two coating layers in order to save money. First the aqueous solution of the usual halophosphate luminescent material is applied onto the inner wall of the discharge tube. Thereafter the organic solvent solution of the luminescent material comprising rare earth metal is applied. All the disadvantageous features of the wet methods occur when this procedure is used.
In order to eliminate the drawbacks of the wet methods, the method of electrostatic coating was worked out. This method is used for the preparation of e.g. low-pressure discharge vessels and described by e.g. U.S. Pat. Nos. 2,426,016 and 4,081,714 and Hungarian patent specification No. 184,030.
According to U.S. Pat. No. 2,426,016 very finely dispersed silicic acid is used in the course of the electrostatic coating of high-pressure mercury vapor discharge tubes in order to enhance the adhesion between the luminescent material and the glass. However, this compound cannot be used for preparing low-pressure discharge tubes wherein the discharge arc directly contacts the luminescent material as the finely dispersed silicic acid is disadvantageous from the point of view of light efficiency and life-time due to its gas permeability and mercury binding ability.
This drawback is aimed to be avoided by U.S. Pat. No. 4,081,714 which shows a process wherein calcium stearate or -palmitate or stearic acid and palmitic acid together with the nitrate of strontium or calcium are used for the surface treatment and adhesion enhancing of luminescent materials, while finely dispersed aluminium oxide is used for assuring the sprayability. The use of calcium stearate together with calcium nitrate binding agent is disadvantageous from the point of sprayability as the latter compound is hygroscopic. The adhesion of the coating has to be assured by vaporization. In the course of this step the bulb having been previously heated to a temperature of 200.degree. C. has to be cooled and thereafter wetted by overheated vapor of about 200.degree. C. temperature. The result of the use of the hygroscopic aluminium oxide can be the same as the use of the finely dispersed silicic acid since the absorbed water cannot be completely removed by the heat treatments generally used in the course of the production of low-pressure mercury vapor discharge tubes. Also, since the salts of stearic or palmitic acid cannot also completely be removed by the heat treatments, it has to be taken account that some contamination will always remain in the coating.
The present invention is intended to eliminate the drawbacks of the above-described methods occuring in the course of the production of low-pressure discharge tubes. Therefore the efforts were directed toward working out such a coating method which on the one hand facilitates the faultless electrostatic application of the luminescent material and results in a perfect quality of the coating, and on the other hand assures good adhesion of the coating to the glass by the aid of the binding agent remaining on the particles of the luminescent material and assures the removal of gas permeable or other disadvantageous substances.